The method of immersion is used for increasing the numerical aperture, and consequently the resolving power, of microscope objectives. In the case of immersion objectives, a viscous liquid (immersion oil) whose refractive index and dispersion match the optical properties of the front lens of the objective is added between the preparation and the objective.
The properties of immersion oils are defined in the standards DIN 58.884 or ISO 8036/1+2. The refractive index of immersion oils which comply with this standard is ne=1.5180±0.0004 at 23° C.
Such immersion oils are described in DE 197 05 978 A1 and DE 197 37 501 A1.
These immersion liquids are introduced between the microscope objective and the sample to be observed in order to increase the resolution of the microscopes. The chemical substances which are used for achieving these high refractive indices, for example chlorinated paraffins, are highly toxic and are, in particular, unsuitable for investigating live biological samples. For this reason, water, which, while only having a refractive index of 1.3339 (at 23° C.) is, on the other hand, biologically tolerated and also possesses very good optical properties (no self fluorescence, high transmission), is generally used for investigating such samples. In order to achieve optimal imaging quality, the microscope objectives have to be matched precisely to the refractive index of the immersion liquid employed, which means that an oil objective cannot be used with water immersion and vice versa. However, when used as an immersion liquid, water suffers from the disadvantage that evaporation occurs, especially in connection with long-term investigations as are increasingly employed in modern biology, with this evaporation then cancelling out the effect of the immersion liquid.
In order to eliminate this problem, DE-OS 101 23 027 proposes a device for automatically supplying immersion liquid. This device has a complicated construction and also suffers from the disadvantage that parts extend into the sample space and thereby impede manipulation of the sample.
The object of the present invention is therefore to design an immersion medium which makes it possible to carry out long-term investigations and which possesses the optical properties of water.